131I-metaiodobenzylguanidine (131I-MIBG) is an active therapy in patients with widely metastatic, treatment-resistant neuroblastoma, where response rates are 20-40% at doses (> 15mCi/kg) requiring stem cell rescue. Irinotecan is a topoisomerase I inhibitor with single-agent chemotherapeutic activity against neuroblastoma and other pediatric solid tumors in phase I and II clinical trials. With more protracted schedules (e.g. daily for 5 days/week x 2 weeks), the major dose-limiting toxicity is diarrhea. With shorter schedules, myelosuppression becomes dose-limiting. In adult solid tumors, irinotecan has been an effective radiosensitizer and is currently being evaluated by the Children's Oncology Group for this purpose in rhabdomyosarcoma protocols incorporating external beam radiotherapy.